P0128: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

Engine is not reaching normal operating temperature.

Low Severity

$150 – $450

Typical repair cost range

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Can I Keep Driving with Code P0128?

Yes, but fuel economy will suffer and heater performance will be weak.

What P0128 Means

Your thermostat is supposed to hold coolant in the engine until it reaches operating temperature (usually 195°F). P0128 means the engine is not warming up fast enough, almost always a stuck-open thermostat.

Common Causes

Stuck-open thermostat (most common)
Low coolant level
Faulty coolant temperature sensor

Diagnostic Tips

Thermostats are usually cheap ($20 to $50) but labor varies widely.
Replace the thermostat gasket and refill with correct coolant type.

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Frequently Asked Questions

Can I just ignore P0128?

You can, but you will waste fuel and the heater may blow lukewarm air in winter.